High integration and high performance of a semiconductor device as typified by a flat panel display using LSIs and a TFT-LCD have been endlessly requested. It is pointed out that surface roughness or an unnecessary semiconductor oxide on a semiconductor surface, an insulating film/semiconductor interface, or an electrode/semiconductor interface brings about deterioration of electrical characteristics of the semiconductor device or a reduction in yield thereof.
A problem with respect to the surface roughness of a gate insulating film/semiconductor interface does not explicitly appear in the case of manufacturing a relatively large-sized device. However, the problem cannot be neglected as a device size has been reduced in recent years and the gate insulating film becomes thinner and thinner. This applies to the unnecessary semiconductor oxide also. The above-mentioned problem becomes more serious with the advent of a next generation device such that a high-dielectric constant material except a silicon oxide film is used for the gate insulating film.
It has been known that the semiconductor surface is oxidized when it is exposed to water and oxygen. According to Non-Patent Document 1, the following has been reported by Morita et al. The oxidation process of silicon is analyzed in an atmosphere. The result of the analysis teaches that oxidation occurs at room temperature after several hours in a usual air atmosphere containing moisture. In contrast to this, oxidation does not occur in a dry air atmosphere containing no moisture even after a lapse of 150 hours or more.
This means that oxidation takes place during a wet process such as cleaning, etching, or chemical mechanical polishing (CMP) each of which uses an aqueous solution Herein, it is noted that oxidation occurring in an aqueous solution is also strongly affected by dissolved oxygen in the liquid. Non-Patent Document 1 describes that the oxidation of silicon in ultra-pure water is strongly affected by dissolved oxygen in the ultra-pure water and that the growth of an oxide film becomes difficult when the concentration of dissolved oxygen is reduced.
In order to suppress the oxidation of silicon during the wet process, use has been made of pure water which has a reduced concentration of dissolved oxygen or which has nitrogen or hydrogen added and dissolved after removal of oxygen.
However, even when the above-mentioned technologies are used, an inevitable problem takes place such that oxygen from the air atmosphere is dissolved in a liquid and oxidation is caused to occur because the wet process such as cleaning, etching, or CMP is carried out in the air atmosphere. In addition, there is another problem that the oxidation of silicon subtly occurs even in hydrogenated water free from oxygen.
With respect to the surface roughness, it has been known that the dissolution of silicon occurs even in pure water with reduced dissolved oxygen and surface flatness deteriorates with the progress of the dissolution. The dissolution rate of silicon increases as pH becomes higher. In the case of a liquid whose pH is equal to or larger than 3, particularly, an alkali solution, roughness occurs and significantly deteriorates the surface flatness.
Many solutions from a neutral range to alkali range, including pure water used to rinse a chemical solution, are often employed in the wet process such as cleaning, etching, or polishing on the semiconductor surface, so the increase in roughness becomes a problem. A cause of the increase in roughness in addition to a cause of the dissolution of silicon are not yet fully determined.
In the case of silicon, it is more likely to cause roughness on a 110 plane or a 551 plane to deteriorate surface flatness as compared with a 100 plane which is a plane orientation used for a conventional semiconductor device. Although the 110 plane is more excellent in electrical device characteristic than the 100 plane, the above-mentioned manufacturing technical problem thereof hinders practical use.
In order to suppress the surface roughness in the wet process described above, there is a method of lowering a pH of an aqueous solution, a method of shortening a treatment time, a method of reducing a temperature, or the like. However, such methods should degrade cleaning performance or etching performance to some extent. Therefore, the elucidation of a roughness increase mechanism and radical improved technologies based thereon have been awaited.    Non-Patent Document 1: J. Appl. Phys., Vol, 68, No. 3: pp. 1272-1281, 1990    Patent Document 1: Japanese Patent (JP-B) No. 3083809